Fires on board aircraft can be extremely damaging, whether to the goods in a cargo hold, the passengers on the aircraft or to the structure of the aircraft itself. It is known in the art to provide aircraft with fire suppression systems, in order to quickly extinguish fires before they can cause serious harm. However, there are a number of problems which make suppressing a fire on board an aircraft quite difficult.
In conventional land-based fire suppression systems, the fire suppression agent may act to starve the fire of oxygen. If a fire has less than 10-12% of oxygen in the surrounding air, it will not restart. However, in aircraft, the various enclosures are not completely airtight and ventilation is provided by a central ventilation system, which circulates air throughout the various enclosures of the aircraft. Some of the fire suppression agent discharged would leak out of the enclosure containing the fire, thereby increasing the proportion of oxygen in the air and possibly allowing a fire to restart.
Additionally, during the descent of an aircraft, the external ambient air and cabin pressures increase, and with that, the proportion of oxygen in the aircraft increases. Thus even if the proportion of oxygen in a protected enclosure is initially reduced below a certain level, over time, the proportion will slowly increase again, thereby undoing the work of the fire suppression agent and potentially causing the fire not to be put out, or to restart. This situation presents a difficulty in controlling the environment around the fire.
The current aircraft fire suppression systems known in the art initially introduce an initial large quantity of fire suppression agent into the enclosure. In order to then avoid the above mentioned problems, these systems then continue to discharge a slow flow of the fire suppression agent, in order to make up for the losses in the aircraft and the addition of further oxygen.
Regarding the choice of fire suppression agent, many current systems make use of halon which may, for example, comprise halon 1211, which is bromochlorodifluoromethane (CF2ClBr), or halon 1301 which is bromotrifluoromethane (CBrF3), or a mixture of the two. However, in recent years, production of halon has become illegal due to environmental concerns of ozone depletion and thus there is a limited supply available for use as a fire suppression agent. Various other fire suppression agents have been tested, including inert gas fire suppression agents. These may include nitrogen, argon, helium, FM 200 or carbon dioxide. There is also the possibility of using recovered nitrogen and carbon dioxide. It has been found that a smaller quantity of halon is required to put out the same size fire than inert gas. Since a substantially greater volume of inert gas needs to carried than that of halon, a greater weight is carried for the same suppressing capability and results in more aircraft fuel being burnt to carry the fire suppression agent. Consequently, for current systems, the environmental impact of the additional greenhouse gases is comparable to the use of halon and so halon is still used in aircraft fire suppression systems, with some systems using a combination of halon and halon-replacement systems.
One of the disadvantages of current aircraft fire suppression systems is that for each new fire, a new bottle of gas is opened to generate the initial high rate release of fire suppression agent, without any consideration as to the remaining contents of the already open bottle. This is wasteful and additionally means that more bottles are carried than may be necessary, thereby resulting in a weight and fuel penalty. Thus there exists in the art a need for an improved aircraft fire suppression system.
It would be desirable to improve the efficiency of inert gas based fire suppression systems so that the weight penalty compared to carrying halon would be reduced and the environmental effects of carrying and using the inert gas could have at least similar, if not less of an environmental impact.
Some improvements in this regard are taught in EP-A-2813266 and EP-A-2353658, the entire contents of which are incorporated herein by reference.